I have recently installed the ArcGIS Server 10.6.1, and only licensed the ArcGIS Image Server component. This is used to publish image data. We purchased the Hexagon GeoSpatial "ECW for ArcGIS Server", and installed as per the directions. When attempting to publish ECW images as image services, the publish succeeds, but the services will not start.
Are there additional steps?
Based on this link, it appears there were additional steps at version 10.
However, this does not install for version 10.6.1.